1. Field of the Invention
This invention relates to a so-called all-weather type pneumatic tire considering performances in a winter season.
2. Description of the Related Art
The all-weather tire is an all-purpose tire capable of running not only on dry and wet road surfaces but also on ice and snow road surfaces. Such a tire is common to have a block pattern formed by arranging on a tread portion straight or zigzag main grooves extending in a circumferential direction of the tire and a plurality of slant grooves crossing these main grooves.
As a means for improving the drainage property, it is useful to take a method of increasing a ratio of groove area or so-called negative ratio by widening a groove width of the each groove. However, there is a problem that as the negative ratio is made large, the rigidity of land portion lowers and hence the satisfactory steering stability is not obtained.
As to the drainage property, the main groove extending in the circumferential direction of the tire develops an effect of draining water in a forward-backward direction of the tire, while the slant groove develops an effect of draining water toward a side of the tire.
As a means for establishing the drainage property and the traction performance on snow in a balanced manner, it is useful to adopt a so-called high-angle groove wherein a groove portion of the slant groove located at a central zone of the tread is extended at a relatively small angle with respect to the circumferential direction of the tire (i.e. as a steeply slant groove) to improve the drainage property and a groove portion located at a side zone of the tread is extended at a relatively large angle with respect to the circumferential direction (i.e. as a gently slant groove) to improve the traction performance on snow.
In a block pattern wherein the steeply slant groove is opened to the main groove located at the central zone, however, a corner part of the block defined between the central main groove and the steeply slant groove becomes acute-angled and the rigidity as a whole of land portions located at the central zone is generally insufficient, and hence the traction performance and steering stability can not be sufficiently ensured and also the uneven wearing is easily caused.
For this end, JP-A-H05-286312 discloses a pneumatic tire wherein the steeply slant groove (portion) is terminated in the land portion located short of the central main groove so as to satisfy both the steering stability and the drainage property (resistance to hydroplaning) at a higher level.
In case of the tread pattern wherein the steeply slant groove (portion) is terminated in the land portion located short of the central main groove as described in the above patent document, however, all of water taken in the central main groove from the central zone in the contacting with ground is drained only toward the forward-backward direction of the tire, so that the drainage property can not be said to be sufficient, and there is still left a room for improving the drainage property.